1. Field of the Invention
The present invention relates to a combustion control system for an internal combustion engine that uses a plurality of fuels having different octane numbers.
2. Description of Related Art
A related art combustion control system for an internal combustion engine includes fuel injection valves that, respectively, inject a low octane fuel and a high octane fuel into a combustion chamber. During the compression stroke, fuel injection is performed such that the low and high octane fuels substantially do not overlap one another in the combustion chamber. The concentration of the gaseous mixture in the combustion chamber is prevented from being over-concentrated, fuel distributions different in octane number can be generated, and stable ignition and suppression of nitrogen oxide and smoke in compressed self ignition are compatibly implemented. Nevertheless, problems described below still remain unresolved.
In the event that distribution of the plurality of fuels having different octane numbers is generated, while preventing the fuels from substantially overlapping one another, fuel ignition is initiated with the low octane fuel. More specifically, compression self-ignition combustion is performed. Since the plurality of fuels having different octane numbers from one another are combusted independently of one another, ignition is facilitated. On the other hand, however, from the viewpoint of controlling combustion, there is a difficulty similar to other conventional cases of conventional compression self-ignition combustion. More specifically, when there is a change in operation conditions, and in particular, when the engine load is high, difficulties in combustion control are known. More specifically, when control is performed only to start ignition with the ignition of the low octane fuel regardless of the engine load, it is difficult to compatibly accomplish high thermal efficiency and output in a wide range of operation conditions.